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                <ol class="chapter"><li class="chapter-item expanded "><a href="ch01-00-get-excited.html"><strong aria-hidden="true">1.</strong> Get excited</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="index.html"><strong aria-hidden="true">1.1.</strong> Check out the README</a></li><li class="chapter-item expanded "><a href="Demo.html"><strong aria-hidden="true">1.2.</strong> See the demo</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-learn-to-use.html"><strong aria-hidden="true">2.</strong> Learn to use Watermelon</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Installation.html"><strong aria-hidden="true">2.1.</strong> Installation</a></li><li class="chapter-item expanded "><a href="Setup.html"><strong aria-hidden="true">2.2.</strong> Setup</a></li><li class="chapter-item expanded "><a href="Schema.html"><strong aria-hidden="true">2.3.</strong> Schema</a></li><li class="chapter-item expanded "><a href="Model.html"><strong aria-hidden="true">2.4.</strong> Defining Models</a></li><li class="chapter-item expanded "><a href="CRUD.html"><strong aria-hidden="true">2.5.</strong> Create, Read, Update, Delete</a></li><li class="chapter-item expanded "><a href="Components.html"><strong aria-hidden="true">2.6.</strong> Connecting to React Components</a></li><li class="chapter-item expanded "><a href="Query.html"><strong aria-hidden="true">2.7.</strong> Querying</a></li><li class="chapter-item expanded "><a href="Relation.html" class="active"><strong aria-hidden="true">2.8.</strong> Relations</a></li><li class="chapter-item expanded "><a href="Writers.html"><strong aria-hidden="true">2.9.</strong> Writers, Readers, batching</a></li></ol></li><li class="chapter-item expanded "><a href="ch03-00-advanced.html"><strong aria-hidden="true">3.</strong> Advanced guides</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Advanced/Migrations.html"><strong aria-hidden="true">3.1.</strong> Migrations</a></li><li class="chapter-item expanded "><a href="Advanced/Sync.html"><strong aria-hidden="true">3.2.</strong> Sync</a></li><li class="chapter-item expanded "><a href="Advanced/CreateUpdateTracking.html"><strong aria-hidden="true">3.3.</strong> Automatic create/update tracking</a></li><li class="chapter-item expanded "><a href="Advanced/AdvancedFields.html"><strong aria-hidden="true">3.4.</strong> Advanced fields</a></li><li class="chapter-item expanded "><a href="Advanced/Flow.html"><strong aria-hidden="true">3.5.</strong> Flow</a></li><li class="chapter-item expanded "><a href="Advanced/LocalStorage.html"><strong aria-hidden="true">3.6.</strong> LocalStorage</a></li><li class="chapter-item expanded "><a href="Advanced/ProTips.html"><strong aria-hidden="true">3.7.</strong> Pro tips</a></li><li class="chapter-item expanded "><a href="Advanced/Performance.html"><strong aria-hidden="true">3.8.</strong> Performance tips</a></li><li class="chapter-item expanded "><a href="Advanced/SharingDatabaseAcrossTargets.html"><strong aria-hidden="true">3.9.</strong> iOS - Sharing database across targets</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-deeper.html"><strong aria-hidden="true">4.</strong> Dig deeper into WatermelonDB</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Implementation/Architecture.html"><strong aria-hidden="true">4.1.</strong> Architecture</a></li><li class="chapter-item expanded "><a href="Implementation/Adapters.html"><strong aria-hidden="true">4.2.</strong> Adapters</a></li><li class="chapter-item expanded "><a href="Implementation/SyncImpl.html"><strong aria-hidden="true">4.3.</strong> Sync implementation</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-deeper.html"><strong aria-hidden="true">5.</strong> Other</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Roadmap.html"><strong aria-hidden="true">5.1.</strong> Roadmap</a></li><li class="chapter-item expanded "><a href="CONTRIBUTING.html"><strong aria-hidden="true">5.2.</strong> Contributing</a></li><li class="chapter-item expanded "><a href="CHANGELOG.html"><strong aria-hidden="true">5.3.</strong> Changelog</a></li></ol></li></ol>
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                        <h1 id="relations"><a class="header" href="#relations">Relations</a></h1>
<p>A <code>Relation</code> object represents one record pointing to another — such as the author (<code>User</code>) of a <code>Comment</code>, or the <code>Post</code> the comment belongs to.</p>
<h3 id="defining-relations"><a class="header" href="#defining-relations">Defining Relations</a></h3>
<p>There's two steps to defining a relation:</p>
<ol>
<li>
<p>A <a href="./Schema.html"><strong>table column</strong></a> for the related record's ID</p>
<pre><code class="language-js">tableSchema({
  name: 'comments',
  columns: [
    // ...
    { name: 'author_id', type: 'string' },
  ]
}),
</code></pre>
</li>
<li>
<p>A <code>@relation</code> field <a href="./Model.html">defined on a <code>Model</code></a> class:</p>
<pre><code class="language-js">import { relation } from '@nozbe/watermelondb/decorators'

class Comment extends Model {
  // ...
  @relation('users', 'author_id') author
}
</code></pre>
<p>The first argument is the <em>table name</em> of the related record, and the second is the <em>column name</em> with an ID for the related record.</p>
</li>
</ol>
<h3 id="immutablerelation"><a class="header" href="#immutablerelation">immutableRelation</a></h3>
<p>If you have a relation that cannot change (for example, a comment can't change its author), use <code>@immutableRelation</code> for extra protection and performance:</p>
<pre><code class="language-js">import { immutableRelation } from '@nozbe/watermelondb/decorators'

class Comment extends Model {
  // ...
  @immutableRelation('posts', 'post_id') post
  @immutableRelation('users', 'author_id') author
}
</code></pre>
<h2 id="relation-api"><a class="header" href="#relation-api">Relation API</a></h2>
<p>In the example above, <code>comment.author</code> returns a <code>Relation</code> object.</p>
<blockquote>
<p>Remember, WatermelonDB is a lazily-loaded database, so you don't get the related <code>User</code> record immediately, only when you explicitly fetch it</p>
</blockquote>
<h3 id="observing"><a class="header" href="#observing">Observing</a></h3>
<p>Most of the time, you <a href="./Components.html">connect Relations to Components</a> by using <code>observe()</code> (the same <a href="./Query.html">as with Queries</a>):</p>
<pre><code class="language-js">withObservables(['comment'], ({ comment }) =&gt; ({
  comment,
  author: comment.author, // shortcut syntax for `author: comment.author.observe()`
}))
</code></pre>
<p>The component will now have an <code>author</code> prop containing a <code>User</code>, and will re-render both when the user changes (e.g. comment's author changes its name), but also when a new author is assigned to the comment (if that was possible).</p>
<h3 id="fetching"><a class="header" href="#fetching">Fetching</a></h3>
<p>To simply get the related record, use <code>fetch</code>. You might need it <a href="./Writers.html">in a Writer</a></p>
<pre><code class="language-js">const author = await comment.author.fetch()

// Shortcut syntax:
const author = await comment.author
</code></pre>
<p><strong>Note</strong>: If the relation column (in this example, <code>author_id</code>) is marked as <code>isOptional: true</code>, <code>fetch()</code> might return <code>null</code>.</p>
<h3 id="id"><a class="header" href="#id">ID</a></h3>
<p>If you only need the ID of a related record (e.g. to use in an URL or for the <code>key=</code> React prop), use <code>id</code>.</p>
<pre><code class="language-js">const authorId = comment.author.id
</code></pre>
<h3 id="assigning"><a class="header" href="#assigning">Assigning</a></h3>
<p>Use <code>set()</code> to assign a new record to the relation</p>
<pre><code class="language-js">await database.get('comments').create(comment =&gt; {
  comment.author.set(someUser)
  // ...
})
</code></pre>
<p><strong>Note</strong>: you can only do this in the <code>.create()</code> or <code>.update()</code> block.</p>
<p>You can also use <code>set id</code> if you only have the ID for the record to assign</p>
<pre><code class="language-js">await comment.update(() =&gt; {
  comment.author.id = userId
})
</code></pre>
<h2 id="advanced-relations"><a class="header" href="#advanced-relations">Advanced relations</a></h2>
<h3 id="many-to-many-relation"><a class="header" href="#many-to-many-relation">Many-To-Many Relation</a></h3>
<p>If for instance, our app <code>Post</code>s can be authored by many <code>User</code>s and a user can author many <code>Post</code>s. We would create such a relation following these steps:-</p>
<ol>
<li>Create a pivot schema and model that both the <code>User</code> model and <code>Post</code> model has association to; say <code>PostAuthor</code></li>
<li>Create has_many association on both <code>User</code> and <code>Post</code> pointing to <code>PostAuthor</code> Model</li>
<li>Create belongs_to association on <code>PostAuthor</code> pointing to both <code>User</code> and <code>Post</code></li>
<li>Retrieve all <code>Posts</code> for a user by defining a query that uses the pivot <code>PostAuthor</code> to infer the <code>Post</code>s that were authored by the User.</li>
</ol>
<pre><code class="language-js">import { lazy } from '@nozbe/watermelondb/decorators'

class Post extends Model {
  static table = 'posts'
  static associations = {
    post_authors: { type: 'has_many', foreignKey: 'post_id' },
  }

  @lazy
  authors = this.collections
    .get('users')
    .query(Q.on('post_authors', 'post_id', this.id));
}
</code></pre>
<pre><code class="language-js">import { field } from '@nozbe/watermelondb/decorators'

class PostAuthor extends Model {
  static table = 'post_authors'
  static associations = {
    posts: { type: 'belongs_to', key: 'post_id' },
    users: { type: 'belongs_to', key: 'user_id' },
  }
  @immutableRelation('posts', 'post_id') post
  @immutableRelation('users', 'user_id') user
}

</code></pre>
<pre><code class="language-js">import { lazy } from '@nozbe/watermelondb/decorators'

class User extends Model {
  static table = 'users'
  static associations = {
    post_authors: { type: 'has_many', foreignKey: 'user_id' },
  }

  @lazy
  posts = this.collections
    .get('posts')
    .query(Q.on('post_authors', 'user_id', this.id));

}
</code></pre>
<pre><code class="language-js">withObservables(['post'], ({ post }) =&gt; ({
  authors: post.authors,
}))
</code></pre>
<hr />
<h2 id="next-steps"><a class="header" href="#next-steps">Next steps</a></h2>
<p>➡️ Now the last step of this guide: <a href="./Writers.html"><strong>understand Writers (and Readers)</strong></a></p>

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